Polycarbonate resins are used in a variety of fields, making use of their impact resistance, transparency, light weight and workability. Particularly, they are used as glass substitutes, making use of their transparency. However, it is known that they are unsatisfactory in terms of weatherability and decompose or deteriorate when they are used outdoors for a long time with the result that their physical properties and appearances are impaired. The surfaces of the polycarbonate resins are readily scratched due to low abrasion resistance and also easily corroded by a solvent.
A trend toward the use of a polycarbonate as organic glass for windows, particularly car windows, making use of their light weight and safety can be seen recently. When a polycarbonate sheet is used for this purpose, weatherability as high as that of glass is required.
Front glass must be protected from scratches by the operation of a wiper and a side window also must be protected from scratches when the window moves vertically. Thus, a high level of abrasion resistance is required for these glasses.
Further, the sunroof is expected to become very hot under a burning sun in the middle of summer. A polycarbonate molded product used for this purpose requires higher durability against environmental variations and a high temperature environment.
To improve these defects, there have been made a large number of proposals for a laminate which comprises a thermosetting acrylic resin layer formed on the surface of a polycarbonate substrate and a siloxane-based cured film formed on the resin layer to improve weatherability, durability and abrasion resistance.
For instance, the inventors of the present invention have proposed the addition of an ultraviolet light absorber having a specific structure to a thermosetting acrylic resin layer in order to improve weatherability (patent document 1). However, weatherability high enough to enable long-term outdoor use still cannot be achieved.
The improvement of durability against environmental variations is proposed (patent document 2). However, due to the low storage stability of a coating, the viscosity of the coating increases during storage or use, thereby making it impossible to use it stably. Further, a molded product having stable physical properties of a coating film cannot be obtained because a side-reaction readily occurs during thermal curing due to the high reactivity of an isocyanate group.
Meanwhile, there is proposed a coating composition which comprises a trihydroxysilane partial condensate and colloidal silica to improve abrasion resistance (patent documents 3 and 4). There is also proposed a coating composition essentially consisting of a partial condensate of an alkyltrialkoxysilane and a tetraalkoxysilane (patent documents 5 and 6).
There is further proposed a coating composition prepared by adding colloidal silica to a condensate of an alkyltrialkoxysilane and a tetraalkoxysilane (patent documents 7 and 8).
However, laminates comprising a cured film obtained from any one of the above coating compositions on a polycarbonate substrate have a certain measure of abrasion resistance but are unsatisfactory in terms of durability against environmental variations and a high temperature environment.
In the above laminates, the coating layer may crack as a thermally cured layer of an organosiloxane resin is pulled under a high temperature environment due to a difference in thermal expansion coefficient between the polycarbonate substrate and the thermally cured layer of an organosiloxane resin.
(patent document 1) JP-A 2000-318106 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)
(patent document 2) JP-A 62-169832
(patent document 3) JP-A 51-2736
(patent document 4) JP-A 55-94971
(patent document 5) JP-A 48-26822
(patent document 6) JP-A 51-33128
(patent document 7) JP-A 63-278979
(patent document 8) JP-A 1-306476
It is an object of the present invention to provide a laminate which has excellent weatherability, durability, abrasion resistance and hot water resistance and which comprises a polycarbonate substrate.
It is another object of the present invention to provide an acrylic resin composition and an organosiloxane resin composition having excellent storage stability, for forming layers on a polycarbonate substrate.
The inventors of the present invention have conducted intensive studies to attain these objects and have found that a laminate which is provided with high levels of weatherability and abrasion resistance and has sufficiently high durability against environmental variations and a high temperature environment and whose surface is protected by a cured film can be obtained by forming a first layer essentially composed of a thermosetting acrylic resin having specific composition on the surface of the polycarbonate substrate and a second layer obtained by thermosetting an organosiloxane resin comprising colloidal silica and a hydrolysis and condensation product of a trialkoxysilane in the mentioned order. The present invention has been accomplished based on this finding.